I do wonder about the demo. All is well and good if it works with no glitches, but what if a spectacular bug catches you out when you don't expect it and the whole thing blows up in your face?
By limiting the scope (rather than the maturity) and testing heavily you can be reasonably confident, but it will leave you with a less impressive demo to throw up.
I think you should view the demo as a much more efficient way to express your vision, rather than thinking of it as a way to impress. The latter doesn't hurt but it isn't as useful towards conveying what YC would like to hear.
By limiting the scope (rather than the maturity) and testing heavily you can be reasonably confident, but it will leave you with a less impressive demo to throw up.